| 摘要 |
A riser in combination with a semispherical ball having a rigid base plate of a given diameter, the riser elevates the ball. The riser includes a portion extending in a longitudinal direction of the riser. The portion has an outside diameter and an inside diameter dimensioned for receiving the given diameter of the rigid base plate. A further portion is adjacent the portion and extends in the longitudinal direction of the riser. The further portion has a further inside diameter. The further inside diameter is smaller than the inside diameter thereby defining a shoulder therebetween. The shoulder is configured for supporting the base plate.
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